While Nd—Fe—B sintered magnets, referred to as Nd magnets, hereinafter, are regarded as the functional material necessary for energy saving and performance improvement, their application range and production volume are expanding every year. Since many applications encounter a hot environment, the Nd magnets incorporated therein must have heat resistance as well as a high remanence. On the other hand, since the coercivity of Nd magnets are easy to decrease significantly at a elevated temperature, the coercivity at room temperature must be increased enough to maintain a certain coercivity at a working temperature.
As the means for increasing the coercivity of Nd magnets, it is effective to substitute Dy or Tb for part of Nd in Nd2Fe14B compound as main phase. For these elements, there are short resource reserves in the world, the commercial mining areas in operation are limited, and geopolitical risks are involved. These factors indicate the risk that the price is unstable or largely fluctuates. Under the circumstances, the development for a new process and a new composition of R—(Fe,Co)—B magnets with a high coercivity, which include a minimizing the content of Dy and Tb, is required.
From this standpoint, several methods are already proposed. Patent Document 1 discloses an R—(Fe,Co)—B base sintered magnet having a composition of 12-17 at % of R (wherein R stands for at least two of yttrium and rare earth elements and essentially contains Nd and Pr), 0.1-3 at % of Si, 5-5.9 at % of B, 0-10 at % of Co, and the balance of Fe (with the proviso that up to 3 at % of Fe may be substituted by at least one element selected from among Al, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, In, Sn, Sb, Hi, Ta, W, Pt, Au, Hg, Pb, and Bi), containing a R2(Fe,(Co),Si)14B intermetallic compound as main phase, and exhibiting a coercivity of at least 10 kOe. Further, the magnet is free of a B-rich phase and contains at least 1 vol % based on the entire magnet of an R—Fe(Co)—Si grain boundary phase consisting essentially of 25-35 at % of R, 2-8 at % of Si, up to 8 at % of Co, and the balance of Fe. During sintering or post-sintering heat treatment, the sintered magnet is cooled at a rate of 0.1 to 5° C./min at least in a temperature range from 700° C. to 500° C. or cooled in multiple stages including holding at a certain temperature for at least 30 minutes on the way of cooling, for thereby generating the R—Fe(Co)—Si grain boundary phase in grain boundary.
Patent Document 2 discloses a Nd—Fe—B alloy with a low boron content. A permanent magnet is prepared from this alloy by sintering a starting material and cooling the sintered product below 300° C. The step of cooling down to 800° C. is at an average cooling rate ΔT1/Δt1<5K/min.
Patent Document 3 discloses an R-T-B magnet comprising a main phase composed mainly of R2Fe14B and a grain boundary phase containing more R than the main phase, the grain boundary phase containing a grain boundary phase having a high rare earth concentration (R-rich phase) and a grain boundary phase having a low rare earth concentration and a high transition metal concentration (transition metal-rich phase). The R-T-B rare earth sintered magnet is prepared by sintering at 800 to 1,200° C. and heat treatment at 400 to 800° C.
Patent Document 4 discloses an R-T-B rare earth sintered magnet comprising a grain boundary phase containing an R-rich phase having a total atomic concentration of rare earth elements of at least 70 at % and a ferromagnetic transition metal-rich phase having a total atomic concentration of rare earth elements of 25 to 35 at %, wherein an area proportion of the transition metal-rich phase is at least 40% of the grain boundary phase. The sintered magnet is prepared by shaping an alloy material into a compact, sintering the compact at 800 to 1,200° C., first heat treatment of heating at a temperature which is in the range of 650 to 900° C. and lower than the decomposition temperature of the transition metal-rich phase, cooling to 200° C. or below, and second heat treatment of heating at 450 to 600° C.
Patent Document 5 discloses an R-T-B rare earth sintered magnet in the form of a sintered body comprising a main phase of R2Fe14B and a grain boundary phase containing more R than the main phase, wherein the main phase has a magnetization direction in c-axis direction, crystal grains of the main phase are of elliptic shape elongated in a direction transverse to the c-axis direction, and the grain boundary phase contains an R-rich phase having a total atomic concentration of rare earth elements of at least 70 at % and a transition metal-rich phase having a total atomic concentration of rare earth elements of 25 to 35 at %. Also described are sintering at 800 to 1,200° C. and subsequent heat treatment at 400 to 800° C. in an argon atmosphere.
Patent Document 6 discloses a rare earth magnet comprising R2T14B main phase crystal grains and an intergranular grain boundary phase between two adjacent R2T14B main phase crystal grains, wherein the intergranular grain boundary phase has a thickness of 5 nm to 500 nm and is composed of a phase having different magnetism from ferromagnetism. It is described that the intergranular grain boundary phase further contains element T and an element which will form a non-ferromagnetic compound. For this purpose, element M such as Al, Ge, Si, Sn or Ga is preferably added. By adding these elements to the rare earth magnet in addition to Cu, a crystalline phase with a La6Co11Ga3-type crystal structure having a good crystallinity may be evenly and broadly formed as the intergranular grain boundary phase, and a thin R—Cu layer may be formed at the interface between the La6Co11Ga3-type intergranular grain boundary phase and the R2T14B main phase crystal grains. As a result, the interface of the main phase is passivated, a lattice distortion of main phase can be suppressed, and nucleation of the magnetic reversal domain can be inhibited. The method of preparing the magnet involves sintering, heat treatment at a temperature in the range of 500 to 900° C. and cooling at a cooling rate which is preferably at least 100° C./min, especially at least 300° C./min.
Patent Document 7 and 8 disclose an R-T-B sintered magnet comprising a main phase of Nd2Fe14B compound, an intergranular grain boundary which is enclosed between two main phase grains and which has a thickness of 5 nm to 30 nm, and a grain boundary triple junction which is the phase surrounded by three or more main phase grains.